Preparations for broadband radio access system are currently underway to aim at a world-wide unified standard. Moreover, there has been desired the establishment of mobile broadband radio access system for next generation, which makes good use of abundant frequency sources, toward submillimeter wave bands.
As the current broadband radio access system, there is used one in which a modulation method is orthogonal frequency division multiplex (OFDM) using a frequency band of 5 GHz under the world-wide unified standard and a modulation multiple value number corresponding to each subcarrier is adaptively controlled according to the circumstances of a propagation path. According to this method, a large modulation multiple value number can be obtained under good circumstances of the propagation path. For this reason, a transmission rate of 54 Mbps using 64-value QAM in, for example, a frequency band of 20 MHz can be attained.
In recent years, application of SDM (Space Division Multiplex) method in which space division multiplex is performed with the same frequency using a plurality of antennas has been considered in order to improve effective use of the frequency (“PDM-COFDM Scheme for Broadband Mobile Communication Systems” Sugiyama, Umehira, The Institute of Electronics, Information and Communication Engineers, Communication Society Meeting, 2001, SB-3-7). In this type of method, the modulation is the same as the conventional case, but different information is transmitted from the plurality of antennas with the same frequency to perform space multiplex. For this reason, for example, in a case where two antennas are used, the transmission capacity is doubled without increasing the frequency band to be used, so that the transmission rate is also doubled.
However, in the aforementioned conventional method, there is a case in which a receiving side cannot perform separation and reproduction of the transmission signal subjected to space multiplex in principle depending on interference from another cell and the circumstances of the propagation path. For this reason, the communication capacity is not always increased, and there is a possibility that such a case will occur where a demand for an expected transmission rate will not be met. In addition, there is a possibility that a communication impossible state will occur depending on the case.